I used Drops (japanese) a few months (874 Words learned until now) and also purchased an one year subscription.
On my Device A I logged in with my Mail-Address and it showed the whole time “Logged in, Everything OK. My Success will be saved.” in the personal menu.
I dropped Device A a few days ago and the display is working but has now the “permanent spider app” installed
Today I got Device B. The subscription got restored.
My Problem is with Backing up and restore my Effort/Success.
On Device B it says, that the User does not exists.
I did also:
login with a complete new e-Mail on Device A in drops.
Tried to login with this new e-Mail on Device B - but also it says: User is unknown.
If I try to “resned the password” it says that the user / client id is unknown.
If I try to reregister the E-Mail - it says that this E-Mail is already registered @_@.

I opened a Ticket: 34962.
The provided solution by Gina was to wirte the Mail-address in lower-case only.
The solution worked for me.
I want to share the solution here because my people read a forum or google for a solution before they contact the support…